Patent GB-584776-A (GB)
Title: Improvements in electrical testing apparatus
Filed: 1944-12-30
Abstract: 584,776. Testing and calibrating instruments; variable contact resistances. BANWELL, C. J. Dec. 30, 1944, No. 26189. [Class 37] In an arrangement for testing potentiometers and the like, the accuracy with which the variation of output with displacement follows a predetermined law is indicated by applying the difference between the output and a potential varying according to the desired law to deflect the trace of a cathode ray tube or recording voltmeter. The potentiometer under test is attached to the plate 1, Fig. 3. adjustable in angular position by the mechanism comprising the knob 9, shaft 8, and worm 7, the worm wheel 5 and attached arm 4 which bears a pin 3 engaging the plate 1. The spindle of the potentiometer is held in the chuck 10 in the shaft 11, which bears a handwheel 15 and pointer 15a at the remote end. The shaft 11 may be rotated manually by a knob (not shown), or driven from the motor 24 by means of a worm (not shown) mounted on the shaft 21, the worm-wheel 19 and the spring ball detent mechanism 20. Also attached to the shaft 11 by means of the insulating bush 40 is the slider arm 39 of the standard potentiometer S which comprises a single turn of resistance wire 32 around the periphery of the anodised aluminium disc 30. The wire 32 is terminated at one end by wedging into a hole drilled in an insulated terminal block 34, Fig. 5, let into the disc 30, and at the other end by attachment by a pin and bush device 36 to the hairpin tensioning spring 37 connected to the second terminal block 35. The arm 39 is connected to the slip-ring 12 through the wiper 41 to the wire 32 through the wiper 38, and by the wiper 49 to an insulating disc 44 carrying a number of interconnected equi-spaced contacts 45. The standard potentiometer Sand the potentiometer T under test are connected across the battery B, Fig. 8, through the variable resistances VR1, VR2 which allow of compensation for varying lengths of resistance element. The output of the standard potentiometer S is applied over a D.C. amplifier X provided with a gain control XGC, a shift control SH and gain switch S2, to the X plates of the cathode ray tube CRT. The differential output of the potentiometers, S, T, is applied over position 4 of the switch S1 to the D.C. amplifier Y provided with gain control YGC and a balance control YB, which supplies deflecting potentials to the Y plates of the tube CRT. The switch S1 provides in position 1 for the input of the amplifier Y to be short-circuited and the balance control YB to be set, in position 2 for the application of a (zero) datum potential, and in position 3 for the application of a calibrating potential from the potentiometer Rl, R3, to the amplifier Y. The face of the tube CRT may bear a datum line a giving the errors of the standard potentiometer S determined by reference to a basic standard, and also .datum lines b, c, giving the permissible tolerances of the potentiometer T. The switch S4 when closed causes the balance of the amplifier Y to be disturbed whenever the slider of the potentiometer S connects with one of the contacts 44, a calibrating pip then occurring in the CRT trace. In an arrangement for testing variable condensers, Fig. 9, the condenser C2 under test is mechanically coupled to a standard condenser C1 and the potentiometer P1, The condensers C1, C2 are connected in the arms of an A.C. bridge circuit comprising the windings 60, 61 of the rransformer TR and the balancing condenser C3. The bridge is driven from an oscillator O and its output taken over an amplifier A1 to a phase-sensitive rectifier R deriving a phase-reference signal from the oscillator O over a phase-shifter PS. The output from the rectifier R is applied over a D.C. amplifier A2 to the Y plates of the cathode ray tube CRT, whose X plates derive a sweeping potential from the potentiometer P1 over the amplifier A3. The arrangement may be employed also for testing a variable inductance against a standard inductance.